This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 10-304079, filed Oct. 26, 1998, the entire contents of which are incorporated herein by reference.
The present invention relates to a polymer electrolyte fuel cell system using an ion conductive solid polymer as an electrolyte.
Recently, fuel cells have been drawn attention by the reason that it may be used as a highly efficient energy conversion apparatus. Of them, attention is particularly focused on a so-called polymer electrolyte fuel cell (PEFC) which is to be used as a power supply source for space machinery and electric vehicles. This is because the PEFC employing a proton exchange membrane, can provide a high power density with a compact structure and be operated in a simple system.
A conventional PEFC system is shown in FIG. 1. The system comprises a polymer electrolyte fuel cell stack 100; a reformer 101 for reforming a fuel, which may be a carbohydrate such as methane and gasoline or an alcohol such as methanol, to produce a hydrogen-rich gas; a shift converter 102 for reducing CO contained in the hydrogen-rich gas; a selective oxidizer 103; a compressor 104 for supplying air as an oxidant; and a cooling-water tank 105 constituting a cooling system for supplying cooling water to remove heat generated by an electrochemical reaction. As the reformer 101, a steam reformer for reforming the fuel with water vapor and an auto thermal reformer are known. The auto thermal reformer is a combination of the steam reformer and a partial-oxidation reformer for reforming the fuel with air. The water vapor required for reforming the fuel is derived from water-vapor of an oxidant gas by separating it by means of a condenser 106. The water vapor is supplied to the reformer 101 by way of the cooling system.
Since the water vapor is recovered by the condenser 106 in the conventional polymer electrolyte fuel cell system, a tank is required for storing the recovered water, namely, pure water, resulting from the electrochemical reaction. The pure water, however, is frozen at 0xc2x0 C. or less. A problem is caused particularly when the fuel cell system is installed in an electric automobile system which must be started up even in a cold region where the automobile system is exposed to an outside air temperature of xe2x88x9240xc2x0 C.
To solve this problem, the following methods have been proposed. In the method disclosed in Japanese Patent Application KOKAI Publication No. 8-185877, an antifreeze is used as the cooling water, whereas pure water is produced by a water separator provided in the cooling system and used as humidifying water. In this method, freezing of the cooling water system is prevented. In addition, pure water can be used for humidification and as a steam for reformer. However, since the pure water is produced by the water separator and supplied by way of a pipe, the pipe is not only frozen at a below-zero environment but also blown out by expansion.
On the other hand, in the method disclosed in Japanese Patent Application KOKAI Publication No. 3-295176, a water absorbent such as calcium chloride or propylene glycol is supplied into an oxidant gas flowing passage within a cell. Water is recovered by the absorbent and then regenerated from the absorbent by a regenerator. In this method, since water is recovered in a vapor state in the oxidant gas flowing passage, a pure-water storage tank is not required. Furthermore, it is possible to minimize freezing of water by lowering the freezing point of the absorbent. However, there is no means for preventing the absorbent from mixing with the oxidant gas. As a result, the absorbent enters an electrode, interfering with an electrochemical reaction. In addition, since the water vapor is contained in an oxidant gas exhaust at a saturated vapor pressure, the generated water is not completely recovered.
The aforementioned problems will not be overcome, even if the methods disclosed in Japanese Patent Application KOKAI Publication Nos. 3-295176 and 8-185877 are combined, in other words, even if an absorbent also serving as an antifreeze is supplied to an oxidant gas flowing passage within the cell to recover water from the oxidant gas and the recovered water is regenerated by a regenerator, while an antifreeze is used as the cooling water, and pure water is produced by the water separator as the reforming vapor.
An object of the present invention is to provide a polymer electrolyte fuel cell system capable of prevent water-freeze within the system even at a below-zero temperature and thereby enabling quick start-up of the system even in a below-zero environment.
To attain the aforementioned object, the present invention comprises
a polymer electrolyte fuel cell stack;
a fuel reformer for reforming a fuel having a freezing point of 0xc2x0 C. or less with water vapor by supplying the fuel and the water vapor to the fuel reformer, the fuel reformed being supplied to the fuel cell stack;
total enthalpy heat exchanging means for exchanging heat and moisture between a reacted gas fed from a reaction section of the fuel cell stack and an unreacted gas to be fed to the reaction section of the fuel cell stack;
separating means for separating the moisture from the reacted gas fed from the total enthalpy heat exchanging means;
mixing means for mixing part of the moisture separated by the separating means into the fuel;
fuel supply means for supplying the fuel to the mixing means; and
solution mixture supply means for supplying part of the solution mixture prepared by the mixing means to the fuel reformer.
Since the separating means and the mixing means are discretely provided in the present invention, the separating means can be reduced in size. Therefore, even if the water stored in the separating means is frozen, the frozen water can be thawed in a short time. It follows that the start-up of the system is not adversely affected. In addition, if the ratio of water to space of the separating means is appropriately controlled, the oxidant gas exhaust can be led out even if water is frozen. The start-up operation of the system is not affected significantly.
Furthermore, since the total enthalpy heat exchanging means is provided for exchanging heat and moisture between the reacted gas fed from the reaction section of the fuel cell stack and the unreacted gas to be fed to the reaction section of the fuel cell stack, the content of the moisture in the exhaust can be drastically reduced. It follows that condensation of water in the exhaust gas flowing passage can be minimized, and water-freeze in the exhaust gas flowing passage can be substantially prevented.
In this case, if the separating means and the mixing means are integrated into one body, heat radiation can be minimized while the temperature of the mixing means can be maintained high. Alternatively, since a partition board is interposed between the separating means and the mixing means, methanol can be prevented from evaporating and entering into the oxidant gas.
Since the present invention employs the total enthalpy heat exchanging means, only one single methanol supply means is sufficient enough. As a result, the system will not be complicated and controlled in a simple manner.
Furthermore, it is possible to prevent the oxidant gas from mixing with undiluted methanol as a fuel. Since the oxidant gas is isolated by pure water from the undiluted methanol as the fuel, virtually no security means is required. As a result, the system will not be complicated and controlled in a simply manner.
To attain the aforementioned object, the present invention comprises
a polymer electrolyte fuel cell stack;
a fuel reformer for reforming a fuel with water vapor by supplying the fuel and the water vapor to the fuel reformer, the fuel reformed being supplied to the fuel cell stack;
total enthalpy heat exchanging means for exchanging heat and moisture between a reacted gas fed from a reaction section of the fuel cell stack and an unreacted gas to be fed to the reaction section of the fuel cell stack;
solution mixture preparing means for preparing a solution mixture of water contained in the reacted gas which is fed from the total enthalpy heat exchanging means and a medium having a freezing point of 0xc2x0 C. or less and a boiling point higher than water; and
water vapor generating means for generating water vapor from the solution mixture prepared by the solution mixture preparing means.
According to the present invention, it is possible to lower the freezing point of the separated moisture by separating the moisture from the fuel gas which is fed from the fuel cell stack or separating from the oxidant gas exhaust and mixing at least part of the separated moisture with a medium which has a freezing point of 0xc2x0 C. or less and a boiling point higher than water. Therefore, freezing of the separated moisture can be prevented. On the other hand, since the medium having a boiling point higher than water is employed, the medium can be prevented from being exhausted from the separator, together with the oxidant gas exhaust. Moreover, since different substances are used as the fuel and the medium, it is possible to prevent freezing of water even in a system employing a water-insoluble fuel such as gasoline.
The solution mixture preparing means may comprise separating means for separating moisture from the gas which is exhausted from the fuel cell stack and mixing means for mixing the medium into part of the water separated from the separating means.
Alternatively, the solution mixture preparing means may comprise the separating means for separating moisture from the gas which is exhausted from the fuel cell stack, and supply means for supplying the medium to the separating means.
Furthermore, the solution mixture preparing means comprises the mixing means for mixing part of the gas exhausted from the medium cell stack, with the fuel.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.